Modern Radio Frequency (“RF”) and microwave communication systems typically utilize digital communication techniques to communicate information. These digital communication techniques include, among others, up-conversion (i.e., a frequency translation method in which the output frequency of an output signal produced by an up-converting device has a higher frequency than the input frequency of an input signal to the up-converting device) and modulation (i.e., a process of varying some characteristics of a carrier wave signal as the information to be transmitted on the carrier wave signal varies). Up-conversion generally includes direct conversion (i.e., the process of frequency translation in a single step) and intermediate frequency (“IF”) up-conversion (i.e., the process of frequency translation with more than a single frequency translation step). Modulation generally includes complex modulation that is also referred to as, for example, “vector modulation,” “quadrature modulation,” “IQ modulation,” “I/Q modulation,” and “I-Q modulation,” where the “in-phase” component of the signal is denoted by I and the “quadrature-phase” component is denoted by Q. Complex modulation generally refers to the independent modulation of the in-phase and quadrature-phase components of a carrier wave signal.
In many situations it is desirable to have RF and/or microwave devices that operate with a wide modulation bandwidth and a calibrated modulation response and are capable of creating complex modulated waveforms on a RF or microwave carrier signal. Generally, vector modulation techniques have wider modulation bandwidth than IF up-conversion techniques, where the modulation bandwidth is generally defined as the maximum rate of change in the output frequency that may be attained utilizing the control voltage of a voltage-controlled frequency source such as, for example, a voltage-controlled oscillator (“VCO”), and the modulation response is generally defined as the frequency dependence of the amplitude modulation imposed on an injected carrier wave signal when the bias current to the voltage controlled frequency source is modulated.
Unfortunately, vector modulators typically suffer from dynamic range impairments (sometime known as “quadrature impairments”) due to various spurious signals. These include gain imbalance, phase imbalance, local oscillator (“LO”) feed-through, and images versus modulation frequency. In addition, the modulation frequency response of the baseband generator as well as the up converter will contribute to complex frequency response variations (amplitude and phase) across the modulation spectrum.
LO feed-through occurs when power at the frequency of the LO used to generate the two sinusoids of the vector modulation process is undesirably present in the output of the quadrature modulator. The power present at the LO frequency disadvantageously wastes valuable output power. Similarly, undesired coupling of the LO into a quadrature demodulator results in an undesirable DC offset in the recovered baseband signals.
Gain imbalance occurs where the two sinusoids have unequal power, when the two baseband signals (input signals to the quadrature modulator, or output signals from the quadrature demodulator) are amplified/attenuated by different amounts by the quadrature modulation or demodulation device or by supporting hardware, e.g., filters and the like. Phase imbalance occurs in situations such as where the two sinusoids used in the quadrature modulation/demodulation process exhibit a phase relationship that deviates from 90-degrees, where there are differences in the group delay between the I and Q circuit paths, and the like.
Attempts at solving these problems have included utilizing direct current (“DC”) calibration methods, scalar spectrum analyzers and power leveling techniques. Unfortunately, these attempted solutions have various shortcomings.
As an example, DC calibration methods have been utilized where DC bias voltages have been applied to a vector modulator and then the resulting up-converted output power was measured as the DC bias voltages were varied. Unfortunately, these types of methods only function at DC. As a consequence there are limitations in the signal to noise ratio (“SNR” or “S/N”) of these types of calibrations because they do not allow the utilization of sophisticated digital signal processing techniques such as, for example, a Fast Fourier Transform (“FFT”). Moreover, these types of methods are fairly slow, do not completely eliminate the spurs, and do not calibrate as a function of the baseband frequency.
As another example, attempts at solving these problems have included utilizing calibration methods that utilize a scalar spectrum analyzer to measure a set of sine wave tones applied the vector modulator. By measuring the scalar frequency response, it is possible to calculate the amplitude response of the up-conversion. Using the amplitude response and certain assumptions, the phase response may also be calculated. However, these types of methods create errors because the characteristics, such as the total delay or delay skew between paired digital-to-analog converter (“DAC”) channels, cannot be determined from a scalar measurement. Unfortunately, these methods require a separate spectrum analyzer, create errors, and are fairly slow.
As a result, there is a need for devices that operate with a wide modulation bandwidth and a calibrated modulation response and are capable of creating complex modulated waveforms on a RF or microwave carrier signal.